126.96.36.199 Persistence and Consistency of InnoDB Transaction and Locking Information
The data exposed by the transaction and locking tables (
INNODB_LOCK_WAITS) represents a glimpse into fast-changing data. This is not like user tables, where the data changes only when application-initiated updates occur. The underlying data is internal system-managed data, and can change very quickly.
For performance reasons, and to minimize the chance of misleading joins between the transaction and locking tables,
InnoDB collects the required transaction and locking information into an intermediate buffer whenever a
SELECT on any of the tables is issued. This buffer is refreshed only if more than 0.1 seconds has elapsed since the last time the buffer was read. The data needed to fill the three tables is fetched atomically and consistently and is saved in this global internal buffer, forming a point-in-time “snapshot”. If multiple table accesses occur within 0.1 seconds (as they almost certainly do when MySQL processes a join among these tables), then the same snapshot is used to satisfy the query.
A correct result is returned when you join any of these tables together in a single query, because the data for the three tables comes from the same snapshot. Because the buffer is not refreshed with every query of any of these tables, if you issue separate queries against these tables within a tenth of a second, the results are the same from query to query. On the other hand, two separate queries of the same or different tables issued more than a tenth of a second apart may see different results, since the data come from different snapshots.
InnoDB must temporarily stall while the transaction and locking data is collected, too frequent queries of these tables can negatively impact performance as seen by other users.
As these tables contain sensitive information (at least
INNODB_TRX.TRX_QUERY), for security reasons, only the users with the
PROCESS privilege are allowed to
SELECT from them.
As described previously, the data that fills the transaction and locking tables (
INNODB_LOCK_WAITS) is fetched automatically and saved to an intermediate buffer that provides a “point-in-time” snapshot. The data across all three tables is consistent when queried from the same snapshot. However, the underlying data changes so fast that similar glimpses at other, similarly fast-changing data, may not be in synchrony. Thus, you should be careful when comparing data in the
InnoDB transaction and locking tables with data in the
PROCESSLIST table. The data from the
PROCESSLIST table does not come from the same snapshot as the data about locking and transactions. Even if you issue a single
PROCESSLIST, for example), the content of those tables is generally not consistent.
INNODB_TRX may reference rows that are not present in
PROCESSLIST or the currently executing SQL query of a transaction shown in
INNODB_TRX.TRX_QUERY may differ from the one in